This invention relates in general to acoustic transducers and more specifically to a high quality speaker where the diaphragm is its own driver.
Conventional speakers for radios, televisions, musical performances, and home hi-fidelity sound systems typically use a cone diaphragm of a flexible sheet material driven at its apex by an electromagnetic coil or a crystal. By selecting the size, shape, material, and mounting of the diaphragm it is possible to tailor the performance of the speaker within wide ranges of performance standards and costs. However, to obtain high quality acoustical performance it is necessary to use materials and techniques that are associated with a high cost. High quality speakers are usually characterized by a low input impedance, large driving currents, and sensitivity to temperature and humidity variations. For example, very light diaphragms are best for high frequency response, but they are difficult to mount and they are sensitive to atmospherics. At the lower performance end, such as conventional automobile and household radio speakers, reduced cost and ruggedness of design are emphasized at the expense of acoustical performance.
Recognizing the limitations of the diaphragm and magnetic drive approach, attempts have been made to drive a diaphragm with piezoelectric crystals. The Motorola company, for example, manufactures a ceramic tweeter formed of barium titinate. Such crystals are also used in microphones and ultrasonic humidifiers.
It is also known to use sheets of a ferroelectric material as a transducer where an applied electrical signal produces a displacement of the ferroelectric material that in turn produces sound waves in the surrounding air, or vice versa. Polyvinyledieneflouride (PVDF) is a dense plastic that exhibits a piezoelectric effect. It is also known to metallize sheets of such a material that have been stretched (in one or more directions), charged (e.g. at 5 KV), and then cooled. The materials, when cooled and shrunk, retains a charge. It expands in the stretched direction in response to an applied positive voltage and contracts in response to an applied negative voltage, also in the stretched direction. Metalized sheets of such ferroelectric materials have been used by Syrix Innovation of Straham, England in contact microphones. Small, dime sized pieces of such a material have been used in ultrasonic humidifiers. Sheets of the material have been used on the hulls of boats to vibrate off material clinging to the hull.
In speaker applications, there have been attempts to use ferroelectric sheets. Heretofore, when the ferroelectric material is used, a foam or resilient backing has been employed to provide structural rigidity. It may also be edge-mounted in a plastic frame. While the frame supports the sheet, it also impedes its movement in response to the applied signal. Also herefore the sheet material has been used with a cylindrical curvature.
Some specific examples of prior attempts to use ferroelectrics include a cylindrical speaker manufactured by Pioneer Electronics. It uses a layer of PVDF laminated on a metal plate. The speaker of this design burned out. With a foam backing it was possible to reach the top two octaves, but the construction would rattle at a low resonant frequency. A novelty item was produced by Pennwalt, a "talking balloon" formed by bonding a metalized PVDF composite onto a rubber backing in the form of a balloon. This construction, however, is not capable of sound reproduction of a quality suitable for ordinary radios, let alone high quality sound systems.
It is therefore a principal object of the present invention to provide a ferroelectric acoustic transducer which uses no magnetic or ceramic driver, exhibits excellent sound quality and is easily driven with no danger of burn out.
Another principal object of this invention is to provide a transducer with the foregoing advantages which has a low cost of manufacture.
A further object of this invention is to provide a transducer with the foregoing advantages which has a low input impedence and low power requirements.
A still further object is to provide a speaker with the foregoing advantages that exhibits a substantially flat response over a broad frequency band normally associated with high quality acoustical speakers.
Yet another object is to provide a transducer with the foregoing advantages that is stable, particularly in response to variations in atmospherics such as temperature and humidity.
Another object of the present invention is to produce a cylindrical sound wave output using essentially a line source.
Still another significant object of the invention is to provide all of the foregoing advantages with a high degree of safety and with a high level of ruggedness.
A further object is to provide a loudspeaker with all of the foregoing advantages that is physically compact and lightweight.